1. Field of the Invention
The present invention relates to substrates having thereon a patterned small molecule organic semiconductor layer. The invention also relates to a method and a system for the production of such substrates having patterned small molecule organic semiconductor layers thereon. The patterned small molecule organic semiconductor layer is formed by the thermal transfer of a small molecule organic semiconductor or its precursor from a donor substrate to an acceptor substrate. More particularly the present invention relates to a substrate having thereon a patterned pentacene semiconductor layer.
2. Description of the Prior Art
The concept of transfer printing of inks and various metals is one that has been known for some time. The general idea involves a substance adhering or affixed to a first surface to be transferred to a second surface with or without direct physical contact between the two surfaces. This type of transfer is well known in non-impact printing of text and also used for transfer of various metals from a first surface to a second surface, usually by way of local pulsed heating of the donor surface. The present invention utilizes preferably a flexible substrate or ribbon or other donor substrates (not necessarily flexible) which contain a small molecule organic semiconducting compound or its precursor that can be locally heated resulting in a transfer, likely by sublimation, of the small molecule organic semiconducting or precursor material from the donor surface resulting in adhesion of the organic semiconductor onto a second or acceptor surface. The transfer of material can be used to define a localized pattern on the acceptor substrate, for example a material that can be used to create a component of a semiconducting device. When the heating method utilizes a focused laser beam, either incident on the semiconducting material or precursor of a small molecule semiconductor such as pentacene, a very narrow and well defined transfer can be achieved to define a part of a semiconducting device, for example the channel of a field effect transistor (FET) or various other semiconducting devices.
Non-impact transfer printing has been used for a variety of applications for a number of years. This form of printing can be divided into two categories; first, one in which there is no contact between the first surface of the donor substrate from which material is transferred and the first surface receiving the material; and second, one in which the two surfaces are in contact but in which there is no impact to impart the material from the first surface of the donor to the first surface of the acceptor. Examples of these types of material transfers are well known in the literature. The transfer of material from a donor surface to an acceptor surface where the acceptor surface may have been previously chemically treated to cause a chemical reaction with the transferred material has been described extensively. For example, ink jet type printers for printing semiconductors and other components of semiconductor devices have been known for some time and are described again most recently in U.S. 2002/0053320 A1.
U.S. Pat. No. 6,344,660 describes impact printing which involves contact between a first and second substrate, wherein the first substrate carries ink or some metal that is to be transferred to the second substrate by local heating, stamping, or spin coating.
When melting is used to transfer material, the source of heat may be a focused laser beam incident on the donor substrate, with a portion of the laser or other energy beam absorbed by the first surface which may be a ribbon. Alternatively, the ribbon may contain an electrically conducting stripe which can be used for localized printing due to the contact of a high resistance element between the contact point on the ribbon and the electrically conducting stripe in which case this form of heating takes the place of the laser to cause the melting and transfer of the material. Print heads that heat the ribbon in one or more places simultaneously are also well known to achieve thermal transfer.
However, there is no prior art known to us in which semiconducting materials are transferred to a second substrate in a crystalline form. In general, to transfer a semiconductor from one surface to a second surface has been achieved by the melting of the material which then is transferred by vaporization from the molten state to a second substrate resulting in an amorphous film. In general, this technique has been widely used in the processing of electroluminescent devices but without the use of a precursor that includes a small organic semiconducting molecule.
It has recently been discovered that small organic semiconducting molecules, such as pentacene, can be thermally transferred from certain substrates (donor substrate) to a second substrate (acceptor substrate) using localized heating. This results in a type of small molecule deposition using energy to provide the thermal energy for the transfer. This type of transfer can be made in a partial vacuum. In more recent experiments it has been found that intimate contact between the donor substrate (containing a pre-deposited small molecule organic semiconductor layer of either the small organic molecule itself or a precursor to that small molecule) and the acceptor substrate yield extremely fine thermally transferred patterns of the small organic molecule onto the acceptor substrate using a focused laser beam. It has been found that the transfer can take place in an ambient atmosphere since the contact between donor and acceptor are sufficiently close to one another that very little, if any of the atmosphere is trapped between the contacting substrates nor can the ambient air enter between the acceptor and donor. This type of intimate contact also precludes the possibility of any substantial contamination of the transferred organic (e. g. pentacene) from the outside ambient.
Thin-film transistors and other electronic devices using organic semiconductors, such as pentacene, are emerging as alternatives to established methods using amorphous silicon (α-Si:H) as the semiconductor.
A variety of organic compounds have been proposed and tested as semiconducting materials for TFT devices. For example, among the p-channel (hole transport) materials that have been characterized are thiophene oligomers proposed as organic semiconductor material for TFT in Garnier, F., et al., “Structural basis for high carrier mobility in conjugated oligomers” Synth. Meth., Vol. 45, p. 163 (1991), and phthalocyanines described in Bao, Z., et al., “Organic Filed-effect transistors with high mobility based on copper phthalocyanine” Appl. Phys. Lett., Vol. 69, p. 3066 (1996). Pentacene, which is a member of poly(acene) compounds has been proposed as an organic semiconductor material in Lin et al. IEEE 54th Annual Device Research Conference, 1996, pages 2136-2139, and Dimitrakopoulos et al., J. Appl. Phys., 80 (4), 1996, pages 2501-2507.
Some soluble organic compounds have also been characterized as organic semiconducting materials. For example poly(3-alkylthiophene) described in Bao, Z., et al., “Soluble and Processable regioregular poly(3-hexylthiophene) for thin film field-effect transistors application with high mobility” Appl. Phys. Lett., Vol. 69, page 4108 (1996).
An attractive material would have a high carrier mobility which is close to that of amorphous silicon (0.1-1 cm2.V−1.s−1), with a very high on/off ratio (>105). For an organic material to replace amorphous silicon would have not only the electrical properties cited above but also should be processable from solution so that it could become commercially attractive.
Among the organic compounds which have been studied as semiconductors, only regioregular poly(3-hexylthiophene) is readily soluble in organic solvents and thin films of this compound have been processed from solution for construction of TFTs. The drawback for this compound is that it has relatively low (5×10−2 cm2.V−1.s.−1) carrier mobility and even much less satisfactory on/off ratio of less than 100. In addition, because thin films of this polymer are not stable in air and its field-effect characteristics deteriorate on exposure to air, its application as semiconductor becomes less desirable.
The best performance as a semiconductor among organic materials to date has been achieved by thin films of pentacene deposited under high vacuum and temperature as reported by Dimitrakopoulos et al., in U.S. Pat. Nos. 5,946,511; 5,981,970 and 6,207,472 and other publications by Brown et al., J. Appl. Phys. 80(4), 1996, pages 2136-2139 and Dimitrakopoulos et al., J. Appl. Phys. 80(4), pages 2501-2507.
Recently, thin-film transistors on plastic substrates using evaporated films of pentacene as the p-channel carrier with mobility of 1.7 cm2.V−1.s.−1 and an on/off ratio of 108 have been reported by Jakson et al., in Solid State Technology, Vol. 43 (3), 2000, pages 63-77.
Thin films of pentacene are very stable in air and even moderate temperatures and as far as performance is concerned, pentacene is probably the most attractive organic material to date to replace amorphous silicon.
The drawback of pentacene is that it is insoluble in common organic solvents and can only be deposited as a thin film by expensive high vacuum and temperature techniques.
There has been very little effort for the synthesis of soluble pentacene derivatives and the only example of soluble pentacene is by Muellen, K. et al., “A soluble pentacene precursor: Synthesis, solid-state conversion into pentacene and application in a field-effect transistor,” Adv. Mat. 11(6), p. 480 (1999), in which a precursor of pentacene is synthesized by a tedious multi-step synthetic approach. The derivative, which is soluble in organic compounds and can be processed from solution, is converted back to pentacene by heating at moderate to high temperatures (140-200° C.).
The drawback for using this compound as a pentacene precursor is that due to the multi-step synthesis (more than 9 steps), its preparation, especially in large scale is impractical. In addition, its conversion to pentacene occurs at a relatively high temperature which prevents the use of most plastics as substrates.
Commonly owned and copending application entitled “Hetero Diels-Alder Adducts of Pentacene as Soluble Precursors of Pentacene,” Ser. No. 10/300,645, Filed on Nov. 20, 2002, IBM ref: YOR920020160US1, contents of which are incorporated herein by reference, describes a specially prepared pentacene precursor that can be spun, dipped, or sprayed onto a substrate from which a small molecule organic semiconductor can result from simple thermal processing of the precursor. The precursor, after application to a substrate, is then allowed to dry. Upon heating the substrate (upon which the dried precursor film resides) on a hot plate at temperatures of 200° C. or less for several minutes or less the precursor has been shown to transform into a pure small molecule organic semiconductor, such as pentacene. Commonly owned and copending application entitled “Thin Film Transistors Using Solution Processed Pentacene Precursor as Organic Semiconductor,” Ser. No. 10/300,630, filed on Nov. 20, 2002, IBM ref: YOR 920020161US1, contents of which are incorporated herein by reference, describes the application of a solution processed polycyclic aromatic compound precursors as an organic semiconducting material in thin film transistors.
The present invention describes a method and a system for producing a substrate having thereon a patterned small molecule organic semiconductor layer, and the patterned substrate itself, wherein the patterned small molecule organic semiconductor layer is produced from the thermal transfer of the small organic molecule from a donor. The small organic molecule feature distinguishes the present invention from those that have transferred polymer or polymer semiconductors from one surface to a second surface in several ways. There is interest in the use of small molecule organic semiconductors in manufacturing items such as light emitting diodes, photodiodes, and field effect transistors (FET's). The present invention provides a cost savings from the usual method of semiconductor device production which normally employs expensive lithographic processes here circumvented by the present invention.
Organic semiconductors are generally cheaper to produce for these applications and are also easier to process as they can be deposited at low temperatures. In addition, this widens the choice of possible substrates including flexible ones that are available in large areas such as MYLAR™ and KAPTON™.
The prior art does not disclose the transfer of small molecules using a simple process of thermal transfer to form patterned layers of the small organic molecule semiconductor material in crystalline form on a substrate.
Accordingly, it is an object of the present invention to provide a method and a system for producing a substrate having thereon a patterned small molecule organic semiconductor layer produced by exposing a donor substrate having thereon a small molecule organic semiconductor layer to energy thus causing the thermal transfer of the small organic molecule onto an acceptor substrate to form the patterned small molecule organic semiconductor layer thereon. The principal application addresses the manufacture of organic field effect transistors (FET's) and organic light emitting diodes on a large scale that that is essentially automated. However, the apparatus and method are not limited to pentacene and can have applications to organic compounds other than pentacene, especially small organic semiconductor molecules.